ch 20; shaping processes for plastics

Question 1

3 reasons plastic shaping processes are important

Answer 1

1) many of the processes are net shape processes
2) less energy is employed than in metalworking processes
3)lower temperatures needed to process plastics than metals or ceramics
4) greater flexibility in geometry
5) painting and other finishing processes not required generally

Question 2

plastics require lower temperatures for processing compared to {} or {}. This is since plastics are composed of { p c}, that soften and become {} at relatively low temps.

Answer 2

metals, ceramics
long polymer chains
moldable

Question 3

common thermoplastics like {} can be processed between around {}°C, while metals often require temperatures above {}°C to melt, and ceramics even higher for {si}

Answer 3

polyethylene, polypropylene
150-250
1000
sintering

Question 4

plastics often have greater {} in geometry due to their moldability and {…} available. Processes such as {}, {} and extrusion allow for the creation of complex shapes with {i d }. Plastics can easily be molded into thin walls and {i d} that would be challenging for metals and ceramics. This flexibility makes plastics ideal from intricate {} devices and large { }

Answer 4

flexibility
variety of processing techniques
blow molding, injection molding
intricate detail
intricate detail
medical devices
automotive parts

Question 5

4 main categories of plastic shaping processes, as classified by resulting product geometry

Answer 5

1) extrusion
2) molding
3) forming of continuous sheets and films
4) fibres
5) foamed products

Question 6

what 3 parameters does the polymer melt’s viscosity depend upon

Answer 6

1) temperature
2) shear rate
3) molecular weight of polymer

Question 7

how does viscosity of polymer melt differ from most Newtonian fluids?

Answer 7

polymer melt exhibits pseudoplasticity i.e. the viscosity decreases with increasing shear rate
(Newtonian fluids have const velocity regardless of shear rate)

Question 8

why do polymers exhibit pseudoplasticity?

Answer 8

viscosity reduces as shear rate increases, long polymer chains align and disentangle under shear, reducing resistance to flow

Question 9

viscoelastic meaning in context of polymer melt

Answer 9

viscous and elastic properties, cause melt to exhibit memory–tendency to return to previous shape, as exhibited by die swell in extrusion

Question 10

briefly describe plastic extrusion process

Answer 10

polymer melt compressed to flow through die orifice and thus the continuous length of plastic assumes aa cross sectional shape the same as the orifice, approximately

Question 11

identify and describe the three sections of the barrel and screw of an extruder

Answer 11

1) the feed section, feed stock fed from hopper and heated
2) compression section, polymer changes to a viscous fluid
3) metering section, pressure developed to pump the plastic through the die orifice

Question 12

2 functions of the screen pack and breaker plate at the die end of the extruder barrel

Answer 12

1) filter dirt and lumps
2) build pressure
3) straighten the flow and remove memory of the polymer melt

Question 13

3 forms of extruded shapes and corresponding dies

Answer 13

1) solid profiles e.g. rounds and L-shapes
2) sheet and film
3) filaments (continuous fibres)
4) wire and cable coating

Question 14

what do sheet and film dies do

Answer 14

transform extruded molten polymer into thin, malleable masses via pressure into a sheet or film die

Question 15

distinction between plastic sheet and film

Answer 15

thickness
sheet stock thickness> 0.5mm
film stock thickness<0.5 mm thick

Question 16

what’s the blown film process for producing film stock

Answer 16

making thin polyethylene film for packaging
combines extrusion and blowing to produce tube of film
beings with extrusion of molten tube, that’s immediately drawn upward, simultaneously expanded in size by air inflated into it through the die mandrel

Question 17

describe calendering process

Answer 17

producing sheet and film stock out of rubbery or rubbery thermoplastics, initial feedstock passed through series of rolls to work the material and reduce thickness to desired gage

Question 18

example of a rubbery thermoplastic

Answer 18

plasticized PVC

Question 19

most commercially important application in polymer fibres and filaments

Answer 19

textiles

Question 20

difference between fibre and filament

Answer 20

fibre–long, thin stran of material, length»diameter (approx 100 times)
filament–fibre of continuous length

Question 21

most important synthetic fibres…

Answer 21

Polyester first
nylon, acrylics, rayon

Question 22

briefly describe injection molding process

Answer 22

polymer heated to highly plastic state, forced to flow under high pressure into mold cavity, where it solidifies, molding then removed from cavity

Question 23

name the two injection molding machine is divided into two principal components

Answer 23

1) injection unit
2) clamping unit

Question 24

function of gates in injection molds

Answer 24

constrict molten plastic flow into the cavity, which increases shear rate, therefore reducing polymer melt viscosity

Question 25

define shear rate

Answer 25

measure of how quickly a fluid is deformed under shear stress, rate at which adjacent layers of fluid move relative to each other i.e. speed one layer of fluid slides past another

Question 26

advantage of using a three plate mold over a 2 plate mold in injection molding

Answer 26

as mold opens, three plate mold automatically separates molded part(s) from runner system

Question 27

3 defects that occur in plastic injection molding

Answer 27

1) short shots, polymer melt solidifies before filling the cavity
2) flashing- polymer melt squeezed into parting surfaces, around ejection pins, between mold halves
3)sink marks, surface drawn into molding by contraction of internal material
4) weld lines, melt has flowed around a core in mold cavity, met from opposite directions–results in mechanical properties inferior to those in rest of part

Question 28

describe structural-foam molding

Answer 28

injection mold process, gas/gas producing ingredient mixed with polymer melt prior to injection into mold cavity, part has tough outer skin surrounded by foam core

Question 29

differences in equipment and operating procedures between

Answer 29

for thermosets:
1) shorter barrel length
2) lower temps in the barrel
1) and 2) prevent premature curing
3) use of heated mold to cause cross linking of a TS polymer

Question 30

what’s reaction injection molding

Answer 30

mixing two highly reactive liquid ingredients, immediately injecting mixture into a mould cavity where chemical reactions cause solidification

Question 31

For reaction injection molding, the two ingredients form components used in {} activated or mixing activated {} systems

Answer 31

catalyst
thermoset

Question 32

what kind of products are produced from blow molding

Answer 32

to produce hollow, seamless containers like bottles

Question 33

what form of starting material is thermoforming?

Answer 33

thermoplastic sheet or film

Question 34

why are molds more costly in mechanical than pressure or vacuum thermoforming?

Answer 34

mechanical-matching mold halves are required
other thermoforming processes-only one mold form required

Question 35

what are 3 processes by which polymer foams are produced

Answer 35

1) mechanical agitation–mixing liquid resin with air, then hardening the polymer by heat or chemical reaction
2) mixing a physical blowing agent with the polymer, a gas e.g. N2
3) mixing polymer with chemical compounds, called chemical blowing agents

Question 36

For polymer foams, the process of mixing a physical blowing
agent with the polymer - a gas such as {}, which can be {} in the
polymer melt under {}, so that the gas comes out of solution and {} when the pressure is
subsequently reduced

Answer 36

nitrogen, N2
dissolved
pressure
expand

Question 37

chemical compounds, known in the context of creating polymer foams, are called { }, they {} at elevated {} to {} gases such as {} or {} within the melt

Answer 37

chemical blowing agents
decompose
temperatures
release/liberate
CO2, N2

Question 38

3 considerations product designers keep in mind when designing components out of plastics

Answer 38

1) not as strong/stiff as metals, not ideal in applications involving high stresses
2) limited service temperatures
3) thermal expansion is greater than metals, dimensional changes due to temp variations more significant for plastics
4) many plastics degrade in sunlight, other forms of radiation, some in oxygen/ozone atmospheres
5) plastics soluble in many solvents
6) impact resistance generally good, better than many ceramics

Question 39

service temperature meaning

Answer 39

range of temps within which a material can be used without significant property degradation

Question 40

four primary variables that influence injection molding process

Answer 40

1) the molder [company responsible/person overseeing/ managing process]
2) material
3) injection machine
4) mold

Question 41

2 types of mold design

Answer 41

1) cold runner 2 plate mold
2) cold runner 3 plate mold
3) hot runner mold

Question 42

most common type of injection molding machine in industry

Answer 42

hydraulic, three-platern system

Question 43

name 4 materials used in blow molding

Answer 43

HDPE (high density polyethylene)
LDPE (low density polyethylene)
Polystyrene (PS)
Polycarbonate (PC)
thermoplastic elasomers
PVC (polyvinyl chloride)

Question 44

HDPE is a {}, while LDPE is a {}, and epoxy is a {}
(thermoset/plastic)

Answer 44

both thermoplastics ( melted and reshaped multiple times, no significant chemical changes)
epoxy–thermoset

Question 45

HDPE is commonly used in…
while LDPE is used in…

Answer 45

HDPE– plastic bottles
LDPE–plastic bags, containers

Question 46

2 common blow molding processes

Answer 46

1) injection blow molding
2) extrusion blow molding

Question 47

stages of extrusion blow molding

Answer 47

1) plasticize resin
2) parison production, or preform production
3) parisono preform inflation and cooling
4) ejection from blow mold
5) finishing and trimming

Question 48

plasticizing resin means…

Answer 48

heating and melting plastic resin, to molten ready for molding (involves mixing resin with plasticizers to enhance flow properties and flexibility)

Question 49

what’s meant by parison production

Answer 49

step in a blow molding process, creates hollow plastic parts e.g. bottles
parison–tube like plastic piece with hole i one end where compressed air can pass

Question 50

what’s meant by electroplating?

Answer 50

finishing operation, coating material surface with thin metal layer (enhances durability)

Question 51

name the four types of finishing operations performed on plastic

Answer 51

degating, deflashing, cleaning, decorating

Question 52

deflashing means…

Why does it occur?

One deflashing technique

Answer 52

removing the excess material (flash) that forms along molds parting lines

molten material seeps into the gaps between mold halves

Grinding, cutting

Question 53

degating means

Answer 53

removing the gate (point where molten plastic enters the mold cavity during injection molding, after plastic part has solidified, the gate remains attached and must be removed to ensure clean finish, can be done so physically)

Question 54

processes to apply decoration to plastic parts

Answer 54

painting, plating (e.g. electroplating) , hot stamping